In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a light source of a shorter wavelength, a projection lens with an increased numerical aperture (NA), and a resist material with improved performance.
With respect to the light source for exposure, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) enabled mass-scale production of DRAM with an integration degree of 64 MB (processing feature size≦0.25 μm). To establish the micropatterning technology (processing feature size≦0.2 μm) necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. Although F2 laser (157 nm) is also considered as one candidate light source of shorter wavelength, the use of F2 laser is postponed because of many outstanding problems including a more expensive scanner.
With respect to the increase of NA, not only an improvement in lens performance is sought for, but also the immersion lithography which can establish an NA of 1.00 or greater by filling a high refractive index liquid between a lens and a wafer is of great interest (see Proc. SPIE, Vol. 5376, p44 (2004)). For the ArF immersion lithography now under investigation, it was proposed to apply to the 45-nm node by filling the space between the lens and the wafer with deionized water having a refractive index of 1.44 (see Proc. SPIE, Vol. 5040, p724 (2003)).
With respect to the resist material, since the development of acid-catalyzed chemical amplification positive working resist materials as disclosed in U.S. Pat. Nos. 4,491,628 and 5,310,619 (JP-B 2-27660 and JP-A 63-27829), it has become possible to achieve a higher resolution and sensitivity. They now become predominant resist materials adapted for deep UV lithography. Of these, the KrF resist materials enjoyed early use on the 0.3 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule. The ArF resist is expected to enable miniaturization of the design rule to 0.13 μm or less.
Various alkali-soluble resins are used as the base resin in such chemically amplified resist compositions. Depending on a light source selected for light exposure, a base resin of different skeleton is used. For KrF resists, a polyhydroxystyrene resin having phenolic hydroxyl groups as the alkali-soluble group is now a standard base resin. On the other hand, for ArF resists, since polyhydroxystyrene resins and novolac resins have very strong absorption at a wavelength around 193 nm, studies were made on poly(meth)acrylate resins and resins comprising cycloaliphatic olefin such as norbornene as polymerized units, both using carboxyl groups as the alkali-soluble group (see JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198). Of these, the poly(meth)acrylate resins are expected to reach a practical level because of ease of polymerization. One of the poly(meth)acrylate resins proposed thus far is a poly(meth)acrylate resin having methyladamantyl groups as the acid labile group and lactone rings as the adhesive group as disclosed in JP-A 9-90637. Norbornyl lactone is also proposed as an adhesive group, having enhanced etching resistance as disclosed in JP-A 2000-26446 and JP-A 2000-159758.
In the case of ArF resist materials, since carboxyl groups as the alkali-soluble functional group have a higher acidity than phenolic hydroxyl groups and hydrophobic cycloaliphatic groups are contained in the polymer skeleton, it is difficult to control the alkali solubility of the resin. This tends to allow for swelling, resulting in undesired phenomena including pattern collapse, increased line edge roughness, and resides left during development. The swell quantity of resist during development can be electrically determined by the quartz crystal microbalance (QCM) technique. With this technique, it was confirmed in Proc. SPIE, Vol. 3999, p2 (2000) that ArF resist materials comprising cycloolefin polymers as the base resin, especially having carboxyl groups as the adhesive group, undergo noticeable swell.
In order to solve these and other problems, it would be necessary to develop a resin having acidic functional groups with an appropriate alkali solubility like phenolic hydroxyl groups on polyhydroxystyrene used in the KrF resist materials. One functional group with an equivalent acidity to phenolic hydroxyl groups is a fluorinated alcohol represented by the partial structure —C(CF3)2OH as proposed in Journal of Photopolymer Science and Technology, Vol. 5, No. 1, p85 (1992). It was observed that polystyrene and norbornene polymers having this functional group incorporated therein exhibit an equivalent alkali solubility to the polyhydroxystyrene. Regrettably, hexafluoroacetone (b.p. −27° C.) used as the starting reactant to incorporate the partial structure —C(CF3)2OH is very toxic and gaseous at room temperature, and thus quite awkward to handle. There exists a strong need for a polymerizable compound which is easy to synthesize in an industrial manner and which have both a (meth)acrylate structure ensuring ease of preparation (polymerization) into a resist resin and functional groups with an equivalent acidity to phenolic hydroxyl groups. One of such recently proposed compounds is an acrylate having as a pendant a tetrahydropyran ring containing trifluoromethyl and hydroxy groups within the ring as reported in Proc. SPIE, Vol. 5376, p556 (2004).